Multistable networks



Oct. 15, 1957 E. AMATNIEK MULTISTABLE NETWORKS 3 Sheets-Sheet 1 Filed Aug. 4, 1951 FIG.2

FIG. In

UNITY GAIN LINE vol

FIG.5

PIC-3.4

FIG. 7

zsgh

FIG.9

FIG.8

ATTORN E Oct. 15, 1957 Filed Aug. 4, 1951 FIGJB E. AMATN IEK MULTISTABLE NETWORKS 5 Sheets-Sheet 3 UNITY GAIN LINE INVENTOR ATTORNEY United ttes atent Office 2 ,8 10,0 72 Patented Oct. 15, 1 957 MULTISTABLE NETWORKS Application August 4, 1951, Serial No. 240,352

2 Claims. (Cl. 250-27) This invention relates to electronic amplifier networks having a plurality of voltage stable states and more particularly to networks of this type including a positive feedback amplifier whose loop gain alternates around unity as a function of input voltage where the alternation is obtained by means of one or more non-linear circuit elements in the network.

Electric circuits having a plurality of stable states are useful for many purposes. For example, they are useful in counters and computers since they may be used to store information which may be supplied thereto in the form of voltage pulses or otherwise. Heretofore the circuit principally employed in the construction of counters has been the Eccles-Jordan dual triode circuit. This circuit possesses two voltage stable states only, therefore it has been convenient to use the binary system of numbers for counting, each binary place being represented by one dual triode. With such a counter of the prior art, to count to one thousand no less than ten dual triodes are required. Therefore, large scale counters and computers based upon such prior art circuits contain tremendous numbers of tubes with their associated power.

dissipation requirements, are subject to frequent breakdowns due to individual tube failures, and involve large installations and high cost.

According to the present invention, a circuit is provided which may possess three or more voltage stable states..

Thus, a dual triode circuit according to the invention having ten stable states can be used to count to ten, so that three instead of ten dual triodes are required to count to one thousand, with corresponding reduction in power dissipation requirements, physical size and cost.

The invention will now be described in detail with reference to the accompanying drawings in which Fig. la is a block diagram of a multisable voltage system according to the invention;

Fig. 1b is an equivalent circuit of Fig. la, including the effective output and input impedances;

Fig. 2 is a graph of the loop transfer characteristic of the non-linear amplifier employed in the multistable voltage system of Fig. 1 as a function of input voltage;

Fig. 3 is a schematic representation of a non-linear resistance element suitable for use in the networks of the invention;

Fig. 4 is a graph illustrating generally the voltageresistance characteristic of the resistance element of Fig. 3;

Fig. 5 is a schematic diagram of a non-linear attenuator or voltage divider network useful in certain e1nbodiment of my invention and including one resistance element having a characteristic similar to that shown in Fig. 4; i

Fig. 6 is a graph showing the transfer characteristic of the non-linearvoltag'e divider of Fig. 5;

Fig. 7 is a schematic diagram of a non-linear voltage divider including a fixed resistance in series with a plurality of paralleled non-linear resistance branches, differently biased to produce the. desired alternating transfer characteristic;

Fig. 8 is the transfer characteristic of the non-linear voltage divider of Fig. 7;

. Fig. 9 is a schematic diagram of an embodiment of the invention employing an attenuator of the general type shown in Fig. 7 and having six voltage stable states;

Fig. 10 is a rearrangement of the diagram of Fig. 9 for purposes of readier comparison with the block diagram of Fig. 1;

Fig. 11 is the transfer characteristic of the two stage amplifier portion of Figs. 9 and 10;

Fig. 12 is the transfer characteristic of the non-linear,

attenuator portion of Figs. 9 and 10;

Fig. 13 is a graph reproducing the data of Figs. 11 and 12 and showing in addition the combined transfer characteristic of the non-linear amplifiers of Figs. 9 and 10, which is also the loop transfer characteristic when the feedback connections are made;

Fig. 14 is a graph on an enlarged scale showing the loop transfer characteristic of the non-linear network of Figs. 9 and 10;

Fig. 15 is a schematic diagram of a second of my'invention;

Fig. 16 is a graph showing the loop transfer characteristic of the circuit of Fig. 15;

Fig. 17 is a schematic diagram of a third embodiment of my invention; and

Fig. 18 is a graph showing the loop transfer characteristic of the network of Fig. 17.

According to the invention, a multistable voltage systern is produced when a positive feedback D. C. amplifier has a loop gain alternating around the value +1.

Such a positive feedback loop is shown in Fig. 1a where the non-linear amplifier 2 has its output terminals 6 and 7 connected by lines 8 and 9 to input terminals 1 and 3 respectively. The loop transfer characteristic corresponding to the alternating loop gain is shown in Fig. 2, in which the horizontally shaded area represents loop gain greater than one, and the vertically shaded area represents loop gain of less than one so that the 45 line sepaembodiment rating the two areas represents unity loop gain. There will be presently described combinations of elements having this transfer characteristic.

For a clearer conception of the formation'of'multistable states one may consider the circuit as shown in Fig. la. For convenience the circuit of Fig. 1a may be replaced by its equivalent in Fig. 1b consisting of an amplifier of gain A, an equivalentoutput resistance R0 (replacing the combined output and input resistances) and an equivalent input capacitance Cm (replacing the combined output, input, and stray capacitances), and in which there appears across terminals 5 and 7 the generated E. M. F., AEin- The D. C. gain from terminals 5 and 7 to input terminals 1 and 3 of the RoCm combination is one, therefore the D. C. loop gain is likewise A.

The system can have three significant conditions of loop gain A: less than unity, more than unity and unity.

If the gain A is less than unity, the voltage AEm across the terminals 5 and 7 is smaller than the input voltage Em across the input terminals 1 and 3. Under these conditions the input capacity C1 will discharge through the resistance R0 so that the input voltage Em will decrease as long as the gain is less than unity, or until the input voltage reaches zero.

When the gain A is more than unity, the voltage AEm is larger than the input voltage Em so that the capacity C1 is charged through the resistance R0 and the input voltage of the network rises. This process continues regeneratively as long as the gain is more than unity.

When the gain A is unity, the input voltage Em isv equal to voltage AEm, and no current flows to charge a or discharge the input capacity C1. The input voltage therefore retains any value imposed from an outside source across the terminals 1 and 3.

The invention provides for the amplifier a variable loop gain alternating about the value unity as a function of input voltage, as shown in Fig. 2. In term-s of Fig. 2, whenever the input voltage Em is such that the corresponding point on the characteristic lies within the vertically shaded area where the loop gain is less than one, the input voltage will decrease as shown by the dotted arrows until the gain reaches the value unity where the loop transfer characteristic crosses the unity gain line. For example, an input voltage e1, corresponding to point Q1 on the transfer characteristic, would decrease as shown by the dotted arrow. Whenever the input voltage Ein is such that the corresponding point on the characteristic lies within the horizontally shaded area where the loop gain is more than one, the input voltage will increase (either positively or negatively) as shown by the solid line arrows in Fig. 2 until the gain again becomes one. For example, an input voltage e2 corresponding to point Q2 on the transfer characteristic would increase as sh wn by the solid arrow. The points at which the transfer characteristic crosses the unity gain line may be classified into points of stable equilibrium S and points of unstable equilibrium U.

If the input voltage Bin corresponding to any one of the points U on the characteristic of Fig. 2 changes by a small amount, the applicable gain will be such that the input voltage will. continue to move away from the point U in question as shown in Fig. 2. Therefore, the points U are points of unstable equilibrium.

However, if the input voltage Ein corresponding to any one of the points S changes by a small amount, the applicable gain will be such that the input voltage will return to the point S in question. Therefore, the points S are points of stable equilibrium. Thus the system can maintain itself at any one of the input voltage values corresponding to points S.

In a practical amplifier in which the gain must decrease at high input voltages due to tube cut-off characteristics, the number of points of stable equilibrium will be where n is the number of crossovers of the unity gain line made by the loop transfer characteristic of the amplifier. To shift the network from one condition of stability to another the voltage at the input terminals 1 and 3 must be changed by external means by an amount sufficient to move the input voltage beyond the adjacent point of unstable equilibrium.

The construction of an amplifier having a loop transfer characteristic of the type shown in Fig. 2 will now be described.

The invention provides an amplifier having a gain characteristic alternating about the unity gain line by combining with two or more stages of direct coupled. amplification one or more non-linear resistors. The nonlinear resistors of the invention are elements whose resistance is, directly or indirectly, dependent upon the voltage across them, their resistance increasing with the applied voltage difference. For high speed operation it is preferred to employ elements whose resistance is directly dependent on the voltage applied. There may also be employed however elements which are indirectly sensitive to the voltage as a result of their temperature coeffi'cients of resistivity. Such elements, being dependent on temperature, are not instantaneous in their response to changes in the voltages applied to them. Materials exist however, such as tungsten lamps, having such pronounced positive temperature coefiicients of resistivity that resistance elements made therefrom may be utilized in the networks of the invention, provided the speed of operation in accepting and retaining successively different applied voltage levels is not required to be too high. Thus in counting applications for example, a network according to the invention employing temperature sensitive resistance elements may be constructed to accept counts at rates up to the order of ten per second.

A preferred form of voltage sensitive resistance element is illustrated in Fig. 3. In Fig. 3 two crystal diodes 15 are connected in series with opposite polarity. This series combination possesses a relatively low resistance for low voltages of either polarity connected across its terminals and a much higher resistance for larger voltages. Fig. 4 illustrates the general nature of the resistance-voltage characteristic of the variable resistance element of Fig. 3. For small applied voltages of either polarity the resistance of the combination is low. As the applied voltage increasesin either direction above a low voltage characteristic of the element, which may be a fraction of a volt or a few volts, the resistance increases rapidly to a much larger value and remains at such larger value for all higher voltages within the range of voltages useful in the circuits of the invention. The ratio of the high to the low resistance values of the combination may be of the order of ten or more. Thermionic vacuum diodes or other devices suitablefor rectification may also be used as voltage sensitive elements.

Subject to the limitations with respect to speed of response set forth above, Fig. 4 is representative of the behavior of both true voltage sensitive elements and of voltage sensitive elements dependent upon changes in temperature, both of which types of. voltage sensitive element may be used in the networks of the invention.

In certain preferred embodiments of the invention, of which examples are shown in Figs. 9, l0 and 15, nonlinear resistors are employed as elements of voltage dividers in order to cause the loop gain of a positive feedback amplifier circuit to alternate about unity. A nonlinear attenuator is built up according to the invention by combining a fixed resistance with a voltage sensitive resistance of the type described in connection with Fig. 4. A preferred form of such an attenuator employing a true voltage sensitive element is shown schematically in Fig. 5. In Fig. 5 a fixed resistance 17 is shown connected in series with a voltage sensitive resistance, generally indicated at the dashed-line box 19, which may for example comprise two oppositely poled crystal diodes 15 as shown in Fig. 3. The transfer characteristic of Fig. 5, i. e. the voltage Eout appearing across the element 19 as' a function of the. voltage Em applied across the elements 17 and 19, is shown in Fig. 6. For low applied voltages Ein, the voltage Eout across the element 19 is also low so that the element 19 displays a low resistance. Consequently the gain of the circuit is low. For higher voltages Em, the resistance of element 19 is correspondingly increased, resulting in increased slope in Fig. 6 and therefore in increased gain for the circuit.

An amplifier having a transfer characteristic alternating about unity gain line may be produced by combining an attenuator as shown in Fig. 5 having the transfer characteristic of Fig. 6 with an amplifier of two or more stages having a transfer characteristic such as that of Fig. 11. The gain of the amplifier is adjusted so that the resultant transfer characteristic of the combination alternates around unity gain as shown in Fig. 18. When the feedback loop is closed, this system will exhibitthree voltage stable conditions. For each additional voltage stable condition an additional dual diode combination must be added. An'attenuator havingfive such of the element 23 has increased substantially.

dual diodes is shown in Fig. 7. In Fig. 7 a fixed resistance 21 is connected in series with a parallel combination 22 of voltage sensitive elements 23, 25, 27, 29 and 31, each branch of the parallel combination being returned to the terminal of the voltage divider opposite the resistance 21 through a biasing source of suitably chosen value. Because of the different biasing voltages employed, the different branches exhibit a minimum resistance at different input voltages Em.

The transfer characteristic of the divider network of Fig. 7, i. e. plot of the voltage EOut appearing across the parallel combination 22 of non-linear resistance elements against the input voltage Em, is shown in Fig. 8.

Since the element 23 is connected directly between the fixed resistance 21 and the opposite input terminal of the divider, the transfer characteristic of Fig. 8 passes through the origin with minimum slope in the same way as the characteristic of Fig. 6. The bias source 24 for the adjacent element 25 is given such a value that the element 25 is not brought into its conducting, i. e. low resistance, range until after the voltage across the element :23 has increased beyond the level at which the resistance Thus each additional dual diode element properly biased produces an additional cross-over point of the transfer char- ;acteristic and the unity gain line.

If the non-linear attenuator of Fig. 7 having the transfer characteristic of Fig. 8 is combined with an amplifier having a transfer characteristic of the form shown in Fig. 11 into a loop with 100% feedback, the resulting circuit will have a loop transfer characteristic of the form of Fig. 2. It will therefore constitute a multistable system.

An embodiment of a multistable voltage system according to the invention is shown in the circuit of Fig. 9, which is redrawn in slightly different form in Fig. 10. The circuit of Fig. 9 includes a voltage amplifier indicated at the dashed-line box 37 of gain greater than unity and a non-linear voltage divider or attenuator indicated at the dashed-line box 49 having a varying gain below unit. The product of the gains of the circuits 37 and 49 is the loop gain of the network. Since as will be presently shown the loop gain alternates about unity, the connection of circuits 37 and 49 into a loop with positive feedback provides a multistable voltage system according to the invention. Within the dashed-line box 37 of Fig. 9 there are provided two amplifier tubes 39 and 45. The tubes 39 and 45 are direct coupled together by means of resistors 40 and 41, and the tube 45 is direct coupled by means of resistors 46 and 47 to the input of a non-linear attenuator shown within the dashed-line box 49. The resistors 41 and 47 are returned to points of negative bias, and the coupling resistors 40, 41, 46 and 47 are suitably proportioned to provide a desired over-all gain for the two tubes between the input terminals 33 and 34 and output terminals 35 and 36 to the vacuum tube amplifier 37. To provide for the circuit of Fig. 9 an average loop gain of unity so that the transfer characteristic will alternate about the unity gain line, the tubes 39 and 45 are adjusted to have together a gain which for optimum performance is the reciprocal of the average gain of the attenuator 49. However, the system will continue to exhibit multistability as long as the transfer characteristic continues to cross and recross the unity gain line, as previously shown.

For the sake of concreteness only, the performance of the circuit of Figs. 9 and will be described on the basis of numerical assumptions in which the gain of the voltage amplifier 37 is chosen to be ten in its linear range and the average gain of the attenuator 49 is chosen to be one tenth, as shown in Figs. 11 and 12 respectively. The amplifier gain remains at the constant value of ten over a range of input voltages E1 extending from approximately +4 /2 volts to -4 /2 volts, producing proportional output voltages between +45 and 45 volts.

. The non-linear attenuator 49 includes four non-linear resistance elements 51, 53, 55, and 57 respectively connected with bias sources 52, 54, 56 and 58in a parallel combination in series with a fixed resistance 60 to form a voltage divider connected across the output terminals 35 and 36 of the voltage amplifier 37. In practice it will be found more convenient to employ a voltage divider in place of the batteries shown in Figs. 9 and 10 for the bias sources connected to the non-linear resistance elements. Batteries are however entirely operable and have been shown in the drawings for the sake of simplicity.

The non-linear resistance elements 51, 53, 55 and 57 are preferably chosen to be identical, and the bias sources 52, 54, 56 .and 58 are dimensioned to provide equal spacing of the cross-over points for .the transfer characteristic of the attenuator about its average value, as shown in Fig. 12, since this permits maximum variation of circuit parameters as Well as minimum sensitivity to triggering by spurious signals. For the same reason the resistance elements are selected so that the maximum and minimum incremental gains of the attenuator have as their geometric mean the desired average gain of the attenuator which is to be the reciprocal of the gain of the amplifier. In terms of the embodiment of Fig. 9 in which the voltage amplifier 37 comprising tubes 39 and has a gain of ten, an average gain of one tenth is provided for the attenuator 49 by selecting the series resistance 60 to be nine times the geometric mean between the parallel resistance of the four non-linear elements 51, 53, 55 and 57 when none of them is in .its low resistance region and the parallel resistance thereof when one of them is in its low resistance region. The resulting transfer characteristic of theattenuator 49 is illustrated in Fig. 12. The curve of Fig. 12 passes through the origin on a steep portion having a slope of .316. For voltages E2 between approximately 4 and +4 volts applied to the input terminals 35 and 36 of the attenuator, the net voltage across each of the elements 53 and 55 is so great that neither of these elements exhibits its low value of resistance. For input voltages E2 from approximately +4 to +16 volts the parallel resistance of the combination is determined largely by the low resistance value of the element 55, and the curve exhibits a slope of .0316. Similarly for input voltages between 4 and -16 volts the curve shows the same slope .0316 due to the operation of the element 53 in its low resistance region. Altogether the curve of Fig. 12 exhibits four regions of slope .0316, each corresponding to a range of input voltages E2 for which one of the elements 51, 53, 55 and 57 is subjected to a net voltage low enough to permit it to exhibit the low resistance value illustrated in Fig. 4.

When a signal is applied across the input terminals of the voltage amplifier 37, it is amplified tenfold. It is then reduced, on the average, tenfold in the attenuator 49. The over-all average gain from the input of the voltage amplifier 37 to the output of the attenuator is therefore unity since the two gains are the reciprocals of each other. This is illustrated in Fig. 13 where, to suitably altered scales, the curves of Figs. 11 and 12 are separately reproduced together with the loop transfer characteristic of the entire circuit. The curve of Fig. 11 is shown as curve M of Fig. 13, that of Fig. 12 as curve N, and the loop transfer characteristic of the circuit is shown as curve 0. The system curve 0, alternating about the unity gain line, is shown at a larger scale again in Fig. 14.

When the feedback connection is completed from the output of the attenuator 49 to the input of the voltage amplifier 37 via the conductor 50, there results a positive feedback non-linear network of the form generally shown in Figs. 1a and 121 having the loop transfer characteristic of Fig. 14. In terms of Fig. 1b, the resistance R0 is the resistance looking back into the attenuator from terminals 38 and 42. Cm is the capacitance looking into the amplifier from terminals 33 and 34. i

7 By inspection of Fig- 14and upon application of the stability criteria previously established, it is seen that the points P are stable points. Since there are eleven crossover points of the loop transfer characteristic and the unity gain line, there are 6 points of stability.

.Since --the .voltage amplifierandmttenuator are connected=in a closed.loop,..it.makes.nozdifierence what points are chosen for the terminals of the.1network..at:which input voltages are-applied for storage or .at which the plural stable voltages may. be zobserved except. as toithe magnitudeof these voltages.

Fig. 10shows the circuit..of.Fig.19 with .thenetwork terminals:between-the. twosamplifying tubes instead of between'the .outputuof .thecattenuatoraand the input to the first amplifying tube. The feedback. connection may likewise beconsidered tobebetween anyv two successive elements of the loop. In Fig. 10, it. may be helpfulto consider the output .of the tube. 39. as being fed back to the-inputtotube '45. .In their. general shape the curves of Figs. 1114 describethe relative variations of the voltages E1,..E2 and E3, of theFig. 10. .To display numerically the values of E1,.E2.and E3, the curves of Figs. 1113 would be redrawn to showinFig. 11a characteristic for thetube 45 havinga gain of 3.16, in place of the gainof 10 shown in Fig. 11 for the two tubes. Similarly Fig. 12 would be redrawn to represent the characteristic .of theattenuator 4-9 plus the tube. 39. This characteristic would have an average gain.of..3 16, a maximum incremental gain .of unity .and .a minimum incremental gain of .1. In Fig. 13,.curve M, there drawn for the combined gains of tubes 39 and 45, would be redrawn to show a gain of 3.16 for the tube 45 alone. Curve N, shown in Fig. 13 for: the: attenuator 49 alone would be redrawn to show a gain fluctuating about the mean value .316 for the attenuator-49and thetube 39. However the curve 0 for the. system of Fig. 13 would be .unchanged,

as would be the curve of Fig. 14.

The voltage amplifying and attenuating elements can be still otherwise disposed. Fig. shows an embodiment in which two tubes and two separate attenuators are combined in a closed feedbackcircuit providing a total of four voltage stable states. An amplifier tube 70 with a plate load resistance 72 is followed by a linear voltage divider comprising resistors 74, '76 and 78. returned to a source of negative bias. A taponthe resistor 76 applies a suitable fraction of the output signal from tube'70 to a second potentiometer. resistor 80. A selected fraction of the voltage across the resistor 80. is applied to the nonlinear voltage divider generally indicated at the dashedline box 82, which includes a fixed resistance 84 in series with two oppositely poled crystal diodes 86. The pair of diodes 86 is returned to a suitablepotential which may or may not be negative with respect to ground, depending upon the operating conditions in the tube 70 and the dimensioning of the resistors which couple it to the attenuator. The attenuator 82 works into a second amplifying tube 90. Like the tube 70, tube 90, is'direct resistance-coupled by a series of resistors 92, 94, 96, 98 and 100 into a divider 102, similar to divider 82. .The non-linear resistance element of divider 102 (shown as two oppositely poled crystal diodes 87) is returned to a bias similar to thatapplied to the diodes 86 of divider 82. The output voltage of the divider 102' is fed back to the grid of tube 70 via .a feedback connection 105. The general shape of the loop transfer characteristic of the circuit of Fig. 15 is shown in Fig. 16. The shape of vthis characteristic can be observed for example by break- .ing the feedback connection 105 and observingthe voltages ,appearingacross the output of divider 102 as the voltage applied to "the grid of tube 70. ischanged.

The shape of the transfer characteristic in Fig. 16 may be explained qualitatively as-follows: With the feedback connection 105 opened the circuit is so established that with zero volts applied to the input terminals 104 and 106, zero volts appear across the output terminals 114 and 116. When the input voltage is-changed to raise the grid of tube relative to its cathode, the voltage at point 108 on the tsp of resistor declines and that at point 112on the tap of resistor correspondingly increases, raising the output voltage between terminals 114 and 116. Since the tubes are operating on linear portions of their characteristic curves, the output voltage so far as governed by the tubes themselves is proportional to the input voltage, although this condition is not necessary to the multistability provided by the invention.

The biasto-which'the series connection of'crystal diodes $6 in divider 82 is connected is sufiiciently negative with respect to the voltage at point 108 corresponding to zero volts across the input terminais 104 and 106 so that the input voltage can rise substantially before the voltageat the point 108 falls enough to bring the series connection of diodes 86 into its range of low resistance. Since the voltage at point 112 is increasing, the crystal diodes 87 are carried progressively farther away from the range of voltages over which they exhibit low resistance.

When the input voltage at 104 and 106 rises sufiiciently however, the voltage at 103 wiil be depressed to the level at which the diodes 86 begin to show their low value of resistance. Accordingly a smaller fraction of the voltage at 108 is applied to the tube 90, and the over-all gain between input terminals 104 and 106 and output terminals 114 and 116 declines. The range of input voltages for which the crystal diodes 86 of divider 32 exhibit their low value of resistance corresponds to the first portion of low slope of the circuit transfer characteristic in the first quadrant of Fig. 16. The topmost intersection of the circuit transfer characteristic with the unity gain line in the firstquadrant represents decrease in the amplification of tube 70 or =90 or both as they approach saturation.

If conversely the input voltage at terminals 104 and 106 is increased negatively from zero to drive the tube '70 toward the cutoff, the voltage at point 103 will increase, and the divider 82 may be neglected as a nonlinear elernent. The voltage at the point 112 declines however and ultimately reaches a level at which the series connection of crystal diodes 87 shows a low resistance. The range of input voltages at 104 and 106 for which the crystal diodes 87 exhibit a low resistance gives rise to the first portion of the loop transfer characteristic having a low slope in the third quadrant.

When the feedback circuit is completed by connection 105, the voltages across terminals 104 and 106 and terminals 114 and 116 become identical, and the circuit will exhibit stability only at points for which the loop transfer characteristic crosses the unity gain line. The voltage at the terminals 104 and 106 can be changed from one to another of the values which correspond to such points of stability in the same way as has been described above in connection with Figs. 9 and 10.

In the embodiments of Figs. 9 and 15 the non-linear resistance element or elements have been used as voltage dividers connected in cascade with vacuum tube amplifiers to form a closed loop. Multistable systems according to the invention can also be provided by employing such resistors to provide negative feedback about one or more stages of amplification. They thus provide secondary loops of negative feedback in parallel with portions of the positive feedback loop. In Fig. 17 two triodes 120 and 124 are resistance coupled together to form the positive feedback loop widely known as the Eccles-Iordan trigger pair, together with a non-linear resistance element generally indicated at the box 128 connected between their plates. For purposes of illustration the non-linear resistance element is shown again as two'oppositely poled crystal diodes 129.

In the usual manner the grid bias resistors 132 and 134 and coupling resistors 136 and 138 are proportioned so that when tube 120 is brought into conduction by an external signal applied to its grid, tube 124 is driven to cutoif or to a state of low conduction by regenerative action, and vice versa.

The voltage difference at the network terminals 144 and 146 has a stable value of approximately zero corresponding to heavy conduction in the tube 120 and to cutoff or a low level of conduction in tube 124. It has another stable value at which the terminal 146 is negative with respect to the terminal 144 by the amount which corresponds to the reverse phase of conduction in the two tubes, tube 120 being at or close to cutoff and tube 124 carrying a heavy plate current In either of these stable conditions a large difference of potential exists between the plates 137 and 139 of the two tubes so that the non-linear resistance element 128 possesses a high value of resistance.

As long as the two plates differ in voltage by more than the range of voltage over which the element 128 is of low resistance, the circuit behaves in the usual manner and may be shifted from one of the stable voltage states above described toward the other by regenerative amplification in the usual way. When however the plates 137 and 139 approach equal voltage levels, the nonlinear resistance element 128 will show a low value of resistance so that it will act as a short circuit from plate to plate, thereby reducing the loop gain of the circuit. The action of the element 128 may also be viewed as negative feedback applied to each of the tubes 120 and 124 by a conductive connection between their plates. The negative feedback so provided can be readily made sufficient to reduce the loop gain of the circuit to unity or less than unity. It provides therefore a point of stable equilibrium in accordance with the analysis set forth in connection With Figs. 1 and 2 above. As seen in Fig. 18 the loop transfer characteristic of the network crosses the unity gain line at the center of the graph where a stable point exists.

While the invention has been described herein in terms of embodiments employing vacuum tube amplifiers, other devices such as transistors may however be used instead.

I claim:

1. An electrical network capable of sustaining a plurality of stable voltages in excess of two, said network comprising two amplifier tubes having direct current coupling between the output of each and the input of the other, and a voltage divider between the output of one of said tubes and the input of the other, said divider including a fixed resistance in series with a plurality of voltage sensitive resistances connected in parallel, and means to bias said voltage sensitive resistances to a plurality of fixed voltages.

2. An electrical network capable of sustaining a plurality of stable voltages in excess of two, said network comprising two amplifier tubes having D. C. coupling between the output of each and the input of the other, and a voltage divider between the'output of one of said tubes and the input of the other, said divider including a fixed resistance in series with a plurality of voltage sensitive resistances connected in parallel, and means to bias said voltage sensitive resistances to a plurality of fixed voltages, each of said voltage sensitive resistances having a relatively low resistance over a limited range of voltages of either polarity applied thereacross and substantially higher resistance for larger voltages applied thereacross, adjacent voltages of said plurality of fixed voltages difiering by at least half the sum of the ranges of voltage over which the voltage sensitive resistances biased by such adjacent fixed voltages exhibit relatively low resistance.

References Cited in the file of this patent UNITED STATES PATENTS 2,025,775 Rieber Dec. 31, 1935 2,548,901 Moe Apr. 17, 1951 2,552,781 Had-field May 15, 1951 2,598,631 Wood May 27, 1952 2,647,995 Dickinson Aug. 4, 1953 

